Exploiting symmetry of the setup and interaction, VMI-ES allows for simple reconstruction of the three-dimensional momentum distribution from two-dimensional images through inversion algorithms. In the VMI-ES the interaction volume is separated from the gas source by a differential pumping section, allowing for high vacuum for the detection chamber and high signal-to-noise ratio acquisition.
Velocity Map Imaging (VMI) is dated back to 1997, to the development of an electrostatic lens system by Eppink and Parker who used it to carry out momentum-resolved detection of photoelectron and photofragment ion of molecular oxygen . Since then it has become a widely used and a powerful experimental technique applied mostly in physics and physical chemistry to obtain energy- and angle resolved information on ions and electrons resulting from the photoionisation or photofragmentation of atoms, molecules or nanoparticles [2-4].
The VMI-ES has two VMI detector assemblies allowing for detection of electrons and ions in two different energy regimes, as it is detailed in the table below. The VMI-ES can be used in combination with several light sources of ELI ALPS with vertical polarisation.
|Parameters VMI-ES spectroscopy system
|Low energy Energy range (ions and electrons)
|0 - 100 eV
|Low energy Energy resolution
|<2% @ 50eV
|High energy Energy range (ions and electrons)
|0 - 1000eV
|High energy Energy resolution
|<2% @ 500eV (based on simulations)
|Pulsed up to 1 kHz, continuous
|17 Hz (limited by camera acquisition rate, planned upgrade/avg over many shots)
*The combination of the VMI-ES with the light sources of ELI ALPS are subject to availability and technical feasibility. Potential users are strongly suggested to contact responsible personnel on the feasibility of the VMI-ES together with a light source.
The VMI-ES consist of three chambers allowing for high vacuum in the detection chamber and high signal-to-noise ratio acquisition. The gas target beam and the light beam (called “ELI-ALPS beam” in the figure below) propagate perpendicularly. The gas target beam is shooting towards the VMI electrodes and detector to allow for measurements where the effect of beam momentum distribution has to be minimised. The double skimmer system allows for a well collimated gas target beam.
The source of the gas target beam is a piezo valve (Amsterdam Cantilever Piezo Valve, ACPV2 model) with operating frequencies up to 1 kHz, and continuous mode. The collimated gas target beam is then formed by a double skimmer system.
The VMI spectrometer has two detection systems, one operating in the 0-100 eV kinetic energy range (low-energy VMI), and one in the 0-1000 eV kinetic energy range (high-energy VMI). The detector can be used either to detect ions or electrons. Ion time-of-flight (TOF)measurements are also available. The multi-channel plate (MCP) detector can be gated to obtain momentum distribution of particles kinetic energy selectively.